1. Field of the Invention
The present invention relates to a transparent electrical conductor and, more particularly, to a transparent electrical conductor for use as an electrical conductor in a frequency range from DC to radio frequency (RF) that is transparent to visible light, formed with an electrical sheet conductance and a visible light transparency better than known transparent electrical conductors, such as tin-doped indium oxide (ITO).
2. Description of the Prior Art
Various opto-electronic devices require transparent electrical conductors that are conductive in the frequency range from DC to radio frequency (RF) and transparent to visible light. Such transparent electrical conductors are known to be applied to such opto-electronic devices in the form of a coating, and have become known as transparent conductive coating (TCC) materials.
Various applications of such TCC materials are known. For example, such TCC materials are known to be used for electrically resistive heating systems for aircraft windshields, as well as in satellite applications. Solar cells are also known to use such TCC materials. In particular, in solar cells applications, the TCC material is used for conducting solar photon-generated currents from the surface of the solar cells, without causing the solar cell to be obscured. Such TCC materials are also known to be used in various other opto-electronic applications, such as liquid crystal displays, CCD camera sensors and photocopiers, as well as a myriad of other opto-electronic type devices.
Various semiconductor coatings with a relatively wide band gap are known to be used for such TCC materials. Specifically, materials having a band gap greater than the energy of the photons of light passed therethrough are known to be used. For transparency across the entire visible/near-infrared (VNIR) band, materials with band gaps wider than 3 eV are known to be used.
In many known applications of such TCC materials, electrical conductivity for such TCC materials approaching that of metals is required. In order for the material to be electrically conductive, one or more of the electron energy bands of the material must be partially filled. In relatively high conductive materials, a partially filled electron energy band normally dominates the conduction.
The density of carriers in the electron energy band, n, required for a specific conductivity, is given by Equation (1). EQU n=.delta./q.mu., (1)
where
q is the electronic charge, PA1 .mu. is the carrier mobility, and PA1 .delta. is the electrical conductivity.
To obtain a sufficient density of carriers in an electron energy band for the desired conductivity, the material is known to be doped because the Fermi level of the intrinsic (pure) material is normally deep within the band gap. However, doping is known to reduce the transmittance of the material for several reasons. First, the optical absorption of free carriers increases with the increasing concentration of carriers, as generally discussed in "Optical Processes in Semiconductors", by J. I. Pankove, Dover Publications, 1971, p.75. Second doping is known to change the density of states function, producing a tail on the absorption near the band edge, as generally discussed in "Absorption Edge of Impure Gallium Arsenide", by J. I. Pankove, Physical Review A, Vol. 140, 1965, pp. 2059-2065. The increase in absorption as a function of the doping level thus causes a fundamental trade-off in such TCC materials between electrical conductivity and VNIR transmittance.
Tin-doped indium oxide (ITO) is known to be used for such TCC material applications. As generally set forth in "Transparent Conductors--A Status Review", by K. L. Chopra, S. Major, and D. K. Pandya, Thin Film Solids, Vol. 102, 1983, pps. 1-46, such ITO coatings are known to have an electron mobility ranging from 15-40 cm.sup.2 /V-s. In many known commercial and aerospace applications, transparent electrical conductors having a sheet electrical conductance of 1 or less ohms per square and a visible light transparency of 90% or better is required. Because of the high refractive index of these conductive coatings, an optical anti-reflection coating would be needed to achieve this transparency over the visible band. A sheet electrical impedance of one ohm per square of the ITO coating requires a doping concentration of about 2.times.10.sup.21 cm.sup.-3. Unfortunately, such highly doped ITO coatings provide less than approximately 75% VNIR transmittance.